diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -879,7 +879,8 @@
         "onDocumentOnTypeFormatting called for non-added file",
         ErrorCode::InvalidParams));
 
-  Reply(Server->formatOnType(Code->Contents, File, Params.position, Params.ch));
+  Server->formatOnType(File, Code->Contents, Params.position, Params.ch,
+                       std::move(Reply));
 }
 
 void ClangdLSPServer::onDocumentRangeFormatting(
@@ -892,12 +893,15 @@
         "onDocumentRangeFormatting called for non-added file",
         ErrorCode::InvalidParams));
 
-  auto ReplacementsOrError =
-      Server->formatRange(Code->Contents, File, Params.range);
-  if (ReplacementsOrError)
-    Reply(replacementsToEdits(Code->Contents, ReplacementsOrError.get()));
-  else
-    Reply(ReplacementsOrError.takeError());
+  Server->formatRange(
+      File, Code->Contents, Params.range,
+      [Code = Code->Contents, Reply = std::move(Reply)](
+          llvm::Expected<tooling::Replacements> Result) mutable {
+        if (Result)
+          Reply(replacementsToEdits(Code, Result.get()));
+        else
+          Reply(Result.takeError());
+      });
 }
 
 void ClangdLSPServer::onDocumentFormatting(
@@ -910,11 +914,14 @@
         "onDocumentFormatting called for non-added file",
         ErrorCode::InvalidParams));
 
-  auto ReplacementsOrError = Server->formatFile(Code->Contents, File);
-  if (ReplacementsOrError)
-    Reply(replacementsToEdits(Code->Contents, ReplacementsOrError.get()));
-  else
-    Reply(ReplacementsOrError.takeError());
+  Server->formatFile(File, Code->Contents,
+                     [Code = Code->Contents, Reply = std::move(Reply)](
+                         llvm::Expected<tooling::Replacements> Result) mutable {
+                       if (Result)
+                         Reply(replacementsToEdits(Code, Result.get()));
+                       else
+                         Reply(Result.takeError());
+                     });
 }
 
 /// The functions constructs a flattened view of the DocumentSymbol hierarchy.
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -247,18 +247,17 @@
                       Callback<ReferencesResult> CB);
 
   /// Run formatting for \p Rng inside \p File with content \p Code.
-  llvm::Expected<tooling::Replacements> formatRange(StringRef Code,
-                                                    PathRef File, Range Rng);
+  void formatRange(PathRef File, StringRef Code, Range Rng,
+                   Callback<tooling::Replacements> CB);
 
   /// Run formatting for the whole \p File with content \p Code.
-  llvm::Expected<tooling::Replacements> formatFile(StringRef Code,
-                                                   PathRef File);
+  void formatFile(PathRef File, StringRef Code,
+                  Callback<tooling::Replacements> CB);
 
   /// Run formatting after \p TriggerText was typed at \p Pos in \p File with
   /// content \p Code.
-  llvm::Expected<std::vector<TextEdit>> formatOnType(StringRef Code,
-                                                     PathRef File, Position Pos,
-                                                     StringRef TriggerText);
+  void formatOnType(PathRef File, StringRef Code, Position Pos,
+                    StringRef TriggerText, Callback<std::vector<TextEdit>> CB);
 
   /// Test the validity of a rename operation.
   void prepareRename(PathRef File, Position Pos,
@@ -323,11 +322,9 @@
   blockUntilIdleForTest(llvm::Optional<double> TimeoutSeconds = 10);
 
 private:
-  /// FIXME: This stats several files to find a .clang-format file. I/O can be
-  /// slow. Think of a way to cache this.
-  llvm::Expected<tooling::Replacements>
-  formatCode(llvm::StringRef Code, PathRef File,
-             ArrayRef<tooling::Range> Ranges);
+  void formatCode(PathRef File, llvm::StringRef Code,
+                  ArrayRef<tooling::Range> Ranges,
+                  Callback<tooling::Replacements> CB);
 
   const ThreadsafeFS &TFS;
 
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -296,40 +296,46 @@
                                 std::move(Action));
 }
 
-llvm::Expected<tooling::Replacements>
-ClangdServer::formatRange(llvm::StringRef Code, PathRef File, Range Rng) {
+void ClangdServer::formatRange(PathRef File, llvm::StringRef Code, Range Rng,
+                               Callback<tooling::Replacements> CB) {
   llvm::Expected<size_t> Begin = positionToOffset(Code, Rng.start);
   if (!Begin)
-    return Begin.takeError();
+    return CB(Begin.takeError());
   llvm::Expected<size_t> End = positionToOffset(Code, Rng.end);
   if (!End)
-    return End.takeError();
-  return formatCode(Code, File, {tooling::Range(*Begin, *End - *Begin)});
+    return CB(End.takeError());
+  formatCode(File, Code, {tooling::Range(*Begin, *End - *Begin)},
+             std::move(CB));
 }
 
-llvm::Expected<tooling::Replacements>
-ClangdServer::formatFile(llvm::StringRef Code, PathRef File) {
+void ClangdServer::formatFile(PathRef File, llvm::StringRef Code,
+                              Callback<tooling::Replacements> CB) {
   // Format everything.
-  return formatCode(Code, File, {tooling::Range(0, Code.size())});
+  formatCode(File, Code, {tooling::Range(0, Code.size())}, std::move(CB));
 }
 
-llvm::Expected<std::vector<TextEdit>>
-ClangdServer::formatOnType(llvm::StringRef Code, PathRef File, Position Pos,
-                           StringRef TriggerText) {
+void ClangdServer::formatOnType(PathRef File, llvm::StringRef Code,
+                                Position Pos, StringRef TriggerText,
+                                Callback<std::vector<TextEdit>> CB) {
   llvm::Expected<size_t> CursorPos = positionToOffset(Code, Pos);
   if (!CursorPos)
-    return CursorPos.takeError();
-  auto Style = format::getStyle(format::DefaultFormatStyle, File,
-                                format::DefaultFallbackStyle, Code,
-                                TFS.view(/*CWD=*/llvm::None).get());
-  if (!Style)
-    return Style.takeError();
-
-  std::vector<TextEdit> Result;
-  for (const tooling::Replacement &R :
-       formatIncremental(Code, *CursorPos, TriggerText, *Style))
-    Result.push_back(replacementToEdit(Code, R));
-  return Result;
+    return CB(CursorPos.takeError());
+  auto Action = [File = File.str(), Code = Code.str(),
+                 TriggerText = TriggerText.str(), CursorPos = *CursorPos,
+                 CB = std::move(CB), this]() mutable {
+    auto Style = format::getStyle(format::DefaultFormatStyle, File,
+                                  format::DefaultFallbackStyle, Code,
+                                  TFS.view(/*CWD=*/llvm::None).get());
+    if (!Style)
+      return CB(Style.takeError());
+
+    std::vector<TextEdit> Result;
+    for (const tooling::Replacement &R :
+         formatIncremental(Code, CursorPos, TriggerText, *Style))
+      Result.push_back(replacementToEdit(Code, R));
+    return CB(Result);
+  };
+  WorkScheduler.run("FormatOnType", std::move(Action));
 }
 
 void ClangdServer::prepareRename(PathRef File, Position Pos,
@@ -561,21 +567,25 @@
   WorkScheduler.runWithAST("SwitchHeaderSource", Path, std::move(Action));
 }
 
-llvm::Expected<tooling::Replacements>
-ClangdServer::formatCode(llvm::StringRef Code, PathRef File,
-                         llvm::ArrayRef<tooling::Range> Ranges) {
+void ClangdServer::formatCode(PathRef File, llvm::StringRef Code,
+                              llvm::ArrayRef<tooling::Range> Ranges,
+                              Callback<tooling::Replacements> CB) {
   // Call clang-format.
-  format::FormatStyle Style = getFormatStyleForFile(File, Code, TFS);
-  tooling::Replacements IncludeReplaces =
-      format::sortIncludes(Style, Code, Ranges, File);
-  auto Changed = tooling::applyAllReplacements(Code, IncludeReplaces);
-  if (!Changed)
-    return Changed.takeError();
-
-  return IncludeReplaces.merge(format::reformat(
-      Style, *Changed,
-      tooling::calculateRangesAfterReplacements(IncludeReplaces, Ranges),
-      File));
+  auto Action = [File = File.str(), Code = Code.str(), Ranges = Ranges.vec(),
+                 CB = std::move(CB), this]() mutable {
+    format::FormatStyle Style = getFormatStyleForFile(File, Code, TFS);
+    tooling::Replacements IncludeReplaces =
+        format::sortIncludes(Style, Code, Ranges, File);
+    auto Changed = tooling::applyAllReplacements(Code, IncludeReplaces);
+    if (!Changed)
+      return CB(Changed.takeError());
+
+    CB(IncludeReplaces.merge(format::reformat(
+        Style, *Changed,
+        tooling::calculateRangesAfterReplacements(IncludeReplaces, Ranges),
+        File)));
+  };
+  WorkScheduler.run("Format", std::move(Action));
 }
 
 void ClangdServer::findDocumentHighlights(
diff --git a/clang-tools-extra/clangd/unittests/ClangdTests.cpp b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
--- a/clang-tools-extra/clangd/unittests/ClangdTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
@@ -878,7 +878,7 @@
   FS.Files[Path] = Code;
   runAddDocument(Server, Path, Code);
 
-  auto Replaces = Server.formatFile(Code, Path);
+  auto Replaces = runFormatFile(Server, Path, Code);
   EXPECT_TRUE(static_cast<bool>(Replaces));
   auto Changed = tooling::applyAllReplacements(Code, *Replaces);
   EXPECT_TRUE(static_cast<bool>(Changed));
diff --git a/clang-tools-extra/clangd/unittests/SyncAPI.h b/clang-tools-extra/clangd/unittests/SyncAPI.h
--- a/clang-tools-extra/clangd/unittests/SyncAPI.h
+++ b/clang-tools-extra/clangd/unittests/SyncAPI.h
@@ -44,6 +44,9 @@
                                     Position Pos, StringRef NewName,
                                     const clangd::RenameOptions &RenameOpts);
 
+llvm::Expected<tooling::Replacements>
+runFormatFile(ClangdServer &Server, PathRef File, StringRef Code);
+
 std::string runDumpAST(ClangdServer &Server, PathRef File);
 
 llvm::Expected<std::vector<SymbolInformation>>
diff --git a/clang-tools-extra/clangd/unittests/SyncAPI.cpp b/clang-tools-extra/clangd/unittests/SyncAPI.cpp
--- a/clang-tools-extra/clangd/unittests/SyncAPI.cpp
+++ b/clang-tools-extra/clangd/unittests/SyncAPI.cpp
@@ -105,6 +105,13 @@
   return std::move(*Result);
 }
 
+llvm::Expected<tooling::Replacements>
+runFormatFile(ClangdServer &Server, PathRef File, StringRef Code) {
+  llvm::Optional<llvm::Expected<tooling::Replacements>> Result;
+  Server.formatFile(File, Code, capture(Result));
+  return std::move(*Result);
+}
+
 std::string runDumpAST(ClangdServer &Server, PathRef File) {
   llvm::Optional<std::string> Result;
   Server.dumpAST(File, capture(Result));